1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method, and more particularly to an image processing apparatus and image processing method that perform correction of density unevenness of ink.
2. Description of the Related Art
In an inkjet printing apparatus, characters or images are printed by ejecting ink drops onto a printing medium from nozzles that are provided in a printing head. In a state where ink drops are not ejected onto a printing medium, the moisture content of the ink in the nozzle evaporates over time, and the ink becomes concentrated. When ejecting concentrated ink onto a printing medium, dots having a thick density are formed.
The ink concentration often occurs near the ejection port of the nozzles, and when ink is ejected several times from the nozzles, ink is supplied from the ink tank that is not concentrated, and the density returns to the normal ink density. In the case of printing an image having uniform density using ink with this kind of concentration characteristics, when the ink is still concentrated, printing is performed with concentrated ink for a short time after printing starts. Therefore, the image on the end section where printing was started becomes dense, and uneven density occurs.
In order to suppress uneven density due to this kind of concentrated ink, technology is known wherein the change in density is estimated from the continuous amount of time that the nozzle has not been used for printing, and the density is kept fixed by determining the combination with ink having a different density than that used for printing (for example, refer to Japanese Patent Laid-Open No. H11-320864 (1999)).
Moreover, technology is known wherein printing is performed by correcting the density signal by a correction amount according to the amount of time that printing has not been performed (for example, refer to Japanese Patent Laid-Open No. 2002-326347).
However, in the technology that is disclosed in Japanese Patent Laid-Open No. H11-320864 (1999), the dot arrangement is set from the ink density signal for the printing image data, and for each dot, which nozzle of which nozzle array will be used to form the dot, is specified. After the nozzles for forming the dots have been specified, the change in density is estimated for each nozzle by checking the number of times the nozzle has not been used continuously for printing. Therefore, it is necessary to set a combination with ink having a different density, and repeatedly set the dot arrangement, the processing of which takes time.
Moreover, when performing printing using the technology disclosed in Japanese Patent Laid-Open No. 2002-326347, the density is corrected for the value of ink density signal before the dot arrangement is set. Therefore, in the areas for which density correction is performed, the number of dots formed changes. In order to estimate the change in density with maximum accuracy, it is necessary, after the ink density signal value has been corrected, to set the dot arrangement again and correct the density signal by a correction amount according to the amount of time that continuous printing has not been performed. Therefore, it is necessary to repeatedly correct the ink density signal value and set the dot arrangement, which processing takes time.
Furthermore, in the patent literatures above, the change in ink density is estimated from the amount of time that ink is not ejected, so that even when only one drop of ink is ejected, the time is reset. However, the concentration of ink proceeds the longer that ink is not ejected, so that when only one drop of ink is ejected, the concentration may not be recovered. In a form such as in the patent literatures above where the time is reset when one drop of ink is ejected, there is a problem in that the time is reset without the ink concentration being recovered, so correction may not be performed adequately.
Taking the above situation into consideration, the object of the present invention is to provide an image processing apparatus that suitably performs correction of image data based on the amount of ink concentration, even when the ink around a nozzle becomes concentrated due to ink not being ejected from the nozzle.